Add --json flag to all commands (Issue #1)

[Guaiyu11]

Summary

Adds --json flag to add, list, done commands.

Changes

• task.py: --json arg on all subparsers
• commands/*.py: JSON output when flag passed

JSON Format

// list --json
{"success": true, "tasks": [{"id": 1, "description": "...", "done": false}]}

// add --json
{"success": true, "task_id": 1, "description": "..."}

// done --json
{"success": true, "task_id": 1, "description": "..."}

Closes #1
/claim #1